This invention relates generally to an AC load protection circuit for use with an AC driven load that is controlled by a semiconductor or solid state controller, and specifically to a circuit that protects against asymmetric input to the load resulting from a failure of the load controller which may damage the load, and/or result in a safety hazard to the load and the load environment.
It is a well known practice to utilize on/off or proportional controllers with AC inductive loads such as motors and the like as well as with various resistive loads. The use of solid state controllers has, however, been limited because of problems which would occur should the control element fail, resulting in a rectified (DC) current to an inductive load which can cause overheating and damage to the load. Thus, in many applications, utilizing such an inductive load, solid state devices such as the triac or silicon-controlled rectifier have not been used because, should such an element fail, it is possible that the inductive load could receive a rectified current, resulting in fire or other damage. In fact, the most probable failure of such a device is a half wave conduction. Thermal overload protection devices, which may be incorporated into the load itself to protect the equipment from overheating, are generally not sufficient to protect the load equipment from such an asymmetrical power input failure because the thermal switching device, after a number of openings and closures, are often welded together by the pulsing DC rectified current, rendering them ineffective. A typical failure of a solid state control element would be an asymmetrical output to the load resulting in a pulsating DC current reaching the load.
Present day devices capable of showing the wave form output of the control element, and thus displaying a half wave output are totally unsuited and impractical for use as a safety device. For example, a CRT can be used to display AC wave forms and could show a half wave effect from a central element. However, such devices are impractical because of their size, the extreme cost involved, and their power requirements.
The present invention allows for the safe use of a solid state control element by providing a protection circuit that is employed in conjunction with the solid state load controller coupled to an AC load (both proportional or full on/off control) that prevents load damage in spite of controller failure. The present invention is also suitable for multiple applications because of its greatly reduced size, its cost effectiveness, extremely low power requirements, and its capability of being positioned within the overall system at a remote location from the load. This allows for the possibility of multiple sensing operations. The present device can also be used in various frequency environments (frequency of the AC input power) when the circuit elements are properly proportioned. The use of the present invention as a safety device is also not necessarily limited to inductive loads, but may be used for inductive-resistive, resistive loads alone, or reactive loads.